Process for obtaining pentafluoroethane by chlorotetrafluoroethane dismutation

ABSTRACT

A gaseous process for obtaining pentafluoroethane by dismutation of chlorotetrafluoroethane in the presence of a supported catalyst, said catalyst being formed of a mixture of trivalent chromium oxide with at least an alkaline-earth metal oxide selected from Mg, Ca, Sr and Ba.

[0001] The present invention relates to a process which allows to obtainvery pure and with high yields CHF₂—CF₃ (HFC 125) pentafluoroethane.

[0002] HFC 125 is an harmless fluorocarbon for the ozone layer,therefore meeting the requirements of the Montreal Treaty. For thecommercial uses of the compound an high purity is required.

[0003] The possibility to obtain pure pentafluoroethane depends on thetype of impurities which are formed during the synthesis. For exampleCFC 115 (chloropentafluoroethane CF₂Cl—CF₃) is an impurity which can beeliminated with difficult from HFC 125, therefore its presence does notallow to obtain the compound at a very pure level. In order to producepentafluoroethane meeting these requirements, processes must be employedwherein CFC 115 does not form or is formed only in traces.

[0004] The industrial processes to produce HFC 125 usually utilize HCFC124 (tetrafluorochloroethane C₂HF₄Cl) as starting compound. HCFC 124 issubjected to fluorination with HF, on suitable catalyst, or istransformed (by dismutation) into a mixture of HFC 125+HCFC 123(dichlorotrifluoroethane C₂HF₃Cl₂) by operating at a suitabletemperature and in the presence of a catalyst. In the patent applicationWO 95/16654 a process for obtaining pentafluoroethane is describedstarting from a previously obtained gas mixture and containing as maincomponent chlorotetrafluoroethane and lower amounts ofchlorofluorocarbons (CFCs) with two carbon atoms. In a first step CFCsare separated, so as to have HCFC 124 substantially pure for thereaction with HF, in particular free from dichlorotetrafluoroethane(C₂Cl₂F₄) CFC 114, which under these conditions would react forming CFC115. Therefore this patent discloses that in order to obtain pure HFC125 by reacting HF with HCFC 124, the starting compound must bepreviously purified by removing the impurities of chloroflurocarbon C₂(114).

[0005] The dismutation process of HCFC 124 is more suitable with respectto the reaction with HF since the starting compound purity is lesscriticl, further the selectivity is higher. This process has thedrawback that the HCFC 124 conversion into HFC 125 is limited by theincrease of the reaction by-products amounts. See EP 569,832 in the nameof the Applicant.

[0006] The need was felt of a process for producing HFC 125 startingfrom HCFC 124, wherein it was possible to increase the amount of theproduced HFC 125, reducing the amount of impurities in comparison withthe fluorocarbon synthesis described with the known processes.

[0007] It has been surprisingly and unexpectedly found by the Applicantthat it is possible to solve the above problem, and this is an object ofthe invention, using a gaseous process, wherein pentafluoroethane isobtained by dismutation of chlorotetrafluoroethane, in the presence of acatalyst comprising a mixture of trivalent chromium oxide with at leastan alkaline-earth metal oxide selected from Mg, Ca, Sr and Ba.

[0008] The reaction temperature is in the range 150° C.-250° C.,preferably 180° C.-240° C.

[0009] The contact time with the catalyst, determined as the ratiobetween the catalyst volume and that of the gas flow at the workingtemperature and pressure is in the range 5-30 seconds, preferably 10-20seconds.

[0010] The pressure is not critical, but preferably is comprised between1 and 10 bar.

[0011] The reaction is carried out by flowing gaseous HCFC 124,optionally diluted with an inert gas such as for example nitrogen,through the catalyst.

[0012] Preferably the reaction is carried out in a fluidized bed; inthis case the catalyst particles must have sizes suitable for this kindof plant.

[0013] The g atoms ratio between the chromium and the alkaline-earthmetals ranges from 50:1 to 3:1, preferably from 20:1 to 5:1.

[0014] The catalyst is preferably supported.

[0015] Preferably the catalyst support is aluminum fluoride obtainableby alumina fluorination, and having a fluorine content not lower than90%, preferably not lower than 95%, with respect to the stoichiometric.

[0016] Generally the used AlF₃ is mainly formed of gamma phase, asdescribed in FR 1,383,927, and has a surface area generally in the range25-35 m²/g. If the catalyst is used in a fluidized bed the support hasthe granulometry suitable for this kind of reactor, as well known to theskilled in the field.

[0017] In the supported catalyst the sum of the percentages of thecontained chromium and alkaline-earth metal is in the range 5-15% byweight, preferably 10-15%.

[0018] The catalyst is preferably prepared by impregnation of thesupport with an aqueous solution of a soluble chromium and of thealkaline-earth metals salt. The support impregnation can be carried outwith any method known in the prior art, for example with the methodknown as dry impregnation, for example as described in EP 408,005,herein incorporated by reference.

[0019] According to this method the impregnation is carried out bypouring on the support, in sequence, according to the process describedhereinunder, portions of an impregnating solution, such that the globalvolume is not higher than the volume of the aluminum fluoride pores. Thesolution for the impregnation is prepared by dissolving in water therequired amounts of the corresponding salts, preferably chlorides, ofthe trivalent chromium and of the alkaline-earth metals. The solution ispoured in portions on the support, drying at 110° C. for some hoursafter every addition, in order to evaporate the water from the supportpores.

[0020] At the end of the impregnation, the catalyst must be activated:the operation can be directly carried out in the reactor used for thedismutation, by calcining in inert gas current, at the temperature ofabout 400° C. for 4-8 hours and then treating at 360° C. with anhydrousHF for 12-24 hours.

[0021] Some Examples are given for illustrative purposes and they arenot limitative of the employment possibility of the invention.

EXAMPLE 1A

[0022] Preparation of a Chromium/Calcium/AlF₃ Catalyst

[0023] 400 g of aluminum fluoride having pore volume of 0.25 cc/g, andgranulometry suitable to the use in a fluidized bed, are impregnatedagain and again with a total of 240 cc of aqueous solution, containing252.5 g of CrCl₃. 6H₂O and 7.7 g of anhydrous CaCl₂, and subsequentlyactivated as above described.

[0024] The so prepared catalyst contains 10% by weight of chromium and0.5% by weight of calcium.

EXAMPLE 1B

[0025] Dismutation of 124 on the Chromium-Calcium Catalyst of Example 1Aat the Temperature of 200° C.

[0026] 350 g of the catalyst prepared according to Example 1A are placedin a 5 cm tubular Inconel® 600 reactor, equipped with porous septum atits base and electrically heated. The catalyst is heated up to 200° C.in nitrogen flow. At this temperature 2 moles/hour (273 g/h) of amixture of about {fraction (95/5)} by moles of the HCFC 124 and HCFC124a isomers are fed. The gases coming out from the reactor are washedin water to absorb acidity traces, and analyzed by gaschromatographywith thermoconductivity detector. The results of the gaschromatographicanalysis on the reaction mixture are reported in Table 1.

[0027] From the Table it results that the HCFC 124 conversion is equalto 59.7% and that the yield in HFC 125 (defined as produced 125/reacted124) is 51.4%. The produced HFC 125 moles/hour are 0.61. The specificproductivity of the produced HFC 125 (grams) in the time unit/weight ofcatalyst is over 210 (g/Kg catalyst)/hour. The CFC 115 content in thereaction mixture is lower than the detectability limit of thegaschromatographic method (about 100 ppm). The analysis of this impurityis repeated by GC-MS, but it results unmeasurable since still lower thanthe detectability limit (1 ppm).

[0028] HCFC 124 and 1110 are recycled in the process. Therefore thelatter is not considered a process impurity.

EXAMPLE 1C

[0029] Dismutation of HCFC 124 on the Chromium-Calcium Catalyst ofExample 1A at the Temperatures of 220° and 240° C. Respectively

[0030] The dismutation reaction is repeated, following the processdescribed in the previous Example 1B, at the temperatures of 220° C. and240° C. respectively.

[0031] The results are reported in Table 1.

[0032] From the Table it is noted that the CFC 115 amount at thetemperature of 220° is unmeasurable with the thermoconductivitydetector, and that also at the temperature of 240° C. the CFC 115 amountremains lower than 100 ppm. At this temperature the HCFC 124 conversionis of about 80% and the yield as above defined is 54.7%. The producedHFC 125 moles/hour are 0.87. Therefore the specific productivity at 240°C. of the produced HFC 125 (grams) in the time unit/weight of catalystis of about 300 (g/Kg catalyst)/hour.

EXAMPLE 2A

[0033] Preparation of a Chrome-Strontium/AlF₃ Catalyst

[0034] 400 g of aluminum fluoride, having pore volume of 0.25 cc/g, withgranulometry suitable to the use in a fluidized bed, are impregnatedagain and again with a total volume of 240 cc of aqueous solutioncontaining 264 g of CrCl₃. 6H₂O and 15.7 g of SrCl₂. 6H₂O, andsubsequently activated as described in the catalyst preparation method.The so prepared catalyst contains 10% by weight of chromium and 1% byweight of strontium.

EXAMPLE 2B

[0035] Dismutation of HCFC 124 on the Chromium-Strontium Catalyst ofExample 2A at the Temperature of 220° C.

[0036] 300 g of catalyst prepared according to Example 2A are placed inthe previously described tubular reactor and heated to 220° C. innitrogen flow. When this temperature is reached, 2 moles/hour (273 g/h)of a nearly {fraction (95/5)} mixture of the HCFC 124 and HCFC 124aisomers are fed. The gases coming out from the reactor are washed inwater to absorb acidity traces, and analyzed by gaschromatography. Theresults are reported in Table 1.

[0037] From the Table it results that the conversion of HCFC 124 is55.8%. The yield is 51.2%. The produced HFC 125 moles/hour are 0.57. Thespecific productivity as above defined is 228 (g/Kg catalyst) /hour. TheCFC 115 content is unmeasurable with the thermoconductivity detector.The analysis repeated by GC-MS gives a CFC 115 amount of about 55 ppm.The CFC 115/HFC 125 ratio is lower than 200 ppm.

EXAMPLE 2C

[0038] Dismutation of HCFC 124 on the Chromium-Strontium Catalyst ofExample 2A at the Temperature of 240° C.

[0039] The test of Example 2B is repeated at the temperature of 240° C.

[0040] The results are reported in Table 1, from which it results thatat 240° C. the formed CFC 115 amount is 100 ppm, the conversion of HCFC124 is 68.5% and the yield is 52.7%. The produced HFC 125 moles/hour atthis temperature are 0.72. The specific productivity of 125 is therefore289 (g/Kg catalyst)/hour.

EXAMPLE 3A

[0041] Preparation of a Chromium/Magnesium/AlF₃ Catalyst

[0042] 500 g of aluminum fluoride having a pore volume equal to 0.25cc/g, with granulometry suitable to the use in a fluidized bed, areimpregnated again and again with a total volume of 327 cc of aqueoussolution containing 327 g of CrCl₃. 6H₂O and 50 g of MgCl₂. 6H₂O. Thecatalyst is subsequently activated as previously described. The soprepared catalyst contains 10% by weight of chromium and 0.9% by weightof magnesium.

EXAMPLE 3B

[0043] Dismutation of HCFC 124 on the Chromium-Magnesium Catalyst ofExample 3A at the Temperature of 220° C.

[0044] 300 g of the catalyst prepared according to Example 3A are placedin the previously described tubular reactor and heated to 220° C. innitrogen flow. When the catalyst is stabilized at this temperature, 2moles/hour (273 g/h) of a nearly {fraction (95/5)} (molar ratio) mixtureof the HCFC 124 and HCFC 124a isomers are fed. The gases coming out fromthe reactor are washed in water to absorb acidity traces, and analyzedby gaschromatography. The following results are obtained:

[0045] 125 31.4% moles

[0046] 124 38.3% moles

[0047] 123 29.9% moles

[0048] others 0.4% moles

[0049] The HCFC 124 conversion is equal to 61.7%. The yield as abovedefined is 50.9%. The produced HFC 125 moles/hour are 0.63. The CFC115/HFC 125 ratio is 100 ppm (GC-MS analysis). The specific productivityof the 125 is 251 (g/Kg catalyst)/hour.

EXAMPLE 4A (Comparative)

[0050] Preparation of a Chromium/AlF₃ Catalyst

[0051] 400 g of aluminum fluoride having a pore volume 0.25 cc/g, andgranulometry suitable to the use in a fluidized bed, are impregnatedagain and again with a total volume of 420 cc of aqueous solution,containing 275 g of CrCl₃. 6H₂O and subsequently activated as abovedescribed.

[0052] The so prepared catalyst contains 10.5% by weight of chromium.

EXAMPLE 4B (Comparative)

[0053] Dismutation Reaction of HCFC 124 on the Catalyst of Example 4A atthe Temperatures of 180° C., 220° C., 240° C., 260° C. 280° C., 300° C.and 320° C. Respectively

[0054] About 400 g of the catalyst prepared according to Example 4A areplaced in the previously described tubular reactor and heated to thereaction temperature in nitrogen flow. When the catalyst is intemperature, 2 moles/hour (273 g/h) of a nearly {fraction (95/5)}mixture of the HCFC 124 and HCFC 124a isomers are fed. The gases comingout from the reactor are washed in water to absorb acidity traces andanalyzed by gaschromatography. The results obtained at the varioustemperatures are reported in Table 2.

[0055] From the Table it results that at the temperature of 180° C. the124 conversion is 43.5%. The produced HFC 125 moles/hour are 0.45. Thespecific productivity is 135 (g/Kg catalyst)/hour. The content of 115 in125, determined by GC, results <100 ppm.

[0056] By using the same above described experimental conditions,reactions at 220° C., 240° C., 260° C., 280° C., 300° C. and 320° C. arecarried out, in order to increase the conversion.

[0057] From Table 2 it results that by operating at temperatures higherthan 220° C., the obtained HFC 125 moles/hour increase but the {fraction(115/125)} ratio has very high values starting from the temperature of240° C. (0.13%). By lowering the temperature to improve the productpurity also the specific productivity decreases. For example at thetemperature of 220° C. the 124 conversion is 52.8% with an yield in 125of 51%. Therefore the specific productivity of 125 is about 161 g/Kgcatalyst/hour. TABLE 1 Dismutation reaction of HCFC 124 onchromium/calcium and chromium/strontium catalyst at the temperatures of200° C., 220° C. and 240° C. In the Table 123 and 124 are the mixturesof the possible isomers. In the Table the specific productivity of 125is indicated with S.P.₁₂₅ Cr/Ca Catalyst Cr/Sr Catalyst (Ex. 1A) (Ex.2A) Temperatures (° C.) 200 220 240 220 240 (Ex. 1B) (Ex. 1C) (Ex. 1C)(Ex. 2B) (Ex. 2C) compound % moles 125 30.67 30.74 43.78 28.58 36.08 1150.00 0.00 0.01 0.00 0.01 124 40.33 39.55 20.02 44.22 31.50 133 0.00 0.010.05 0.16 0.14 114 0.01 0.02 0.04 0.04 0.07 123 28.72 29.38 34.79 26.7631.48 1110  0.18 0.14 1.00 0.10 0.36 others 0.09 0.15 0.31 0.12 0.30convers. 59.67 60.45 79.98 55.78 68.50 124 % moles 125 0.61 0.62 0.870.57 0.72 moles/h obtained S.P.₁₂₅ 210 212 300 228 289 115/125 <1 ppm<100 ppm 0.02 0.02 0.03 % moles in the in the crude crude productproduct

[0058] TABLE 2 Ex. 4B - comparative: Analysis of the gases contained inthe reaction mixture obtained by dismutation of HCFC 124 in the presenceof a Cr^(III)/ALF₃ catalyst at the temperatures of 180° C., 220° C.,240° C., 260° C., 280° C., 300° C. and 320° C. respectively. S.P.₁₂₅ hasthe same meaning of Table 1. Compound % moles in the Temperatures (° C.)mixture 180 220 240 260 280 300 320 125 22.36 27.01 39.80 45.02 50.6950.28 52.15 115 0.00 0.00 0.05 0.16 0.51 0.95 2.43 124 58.50 47.20 25.5321.77 18.57 19.15 18.22 133 0.03 0.06 0.16 0.31 0.65 1.07 2.23 114 0.020.07 0.10 0.16 0.23 0.44 0.69 123 21.07 25.49 33.39 31.48 27.96 26.3122.13 1110  0.00 0.05 0.68 1.08 0.72 0.60 0.55 Others 0.02 0.11 0.280.04 0.67 1.19 1.6 Conv. 124 41.50 52.80 74.47 78.23 81.43 80.85 81.78125 0.45 0.54 0.79 0.90 1.01 1.00 1.04 moles/h S.P.₁₂₅ 135 161 236 269304 302 313 115/125 <100 ppm <100 ppm 0.13 0.36 1.01 1.88 4.67 % molesin the in the crude crude product product

1. A gaseous process for obtaining pentafluoroethane by dismutation ofchlorotetrafluoroethane, in the presence of a catalyst comprising amixture of trivalent chromium oxide with at least an alkaline-earthmetal oxide selected from Mg, Ca, Sr and Ba.
 2. A process according toclaim 1, wherein the reaction temperature is in the range 150° C.-250°C., preferably 180° C.-240° C.
 3. A process according to claims 1-2,wherein the contact time with the catalyst is in the range 5-30 seconds,preferably 10-20 seconds.
 4. A process according to claims 1-3, whereinthe g atoms ratio between the chromium and the alkaline-earth metalsranges from 50:1 to 3:1, preferably from 20:1 to 5:1.
 5. A processaccording to claims 1-4, wherein the catalyst is supported.
 6. A processaccording to claim 5, wherein the support is aluminum fluorideobtainable by alumina fluorination, having a fluorine content not lowerthan 90%, preferably not lower than 95%, with respect to thestoichiometric.
 7. A process according to claim 6, wherein the aluminumfluoride is mainly formed of gamma phase and has a surface areagenerally in the range 25-35 m²/g.
 8. A process according to claims 5-7,wherein in the supported catalyst the sum of the percentages of thecontained chromium and alkaline-earth metals is in the range 5-15% byweight, preferably 10-15%.